Ferroelastic nature of high pressure phase transition in MgF2

Using quantum theoretical simulations based on density functional and density functional perturbation theory we illuminate the nature of the phase transition occurring in magnesium fluoride (MgF2) at high pressure. At atmospheric pressure it is stable in the rutile structure. It was predicted by theory and later confirmed by experiments that a phonon soft mode driven phase transition transforms MgF2 to a CaCl2-type structure. The phase transition was connected with an optic phonon mode B1g. By simulating the behavior of phonons at and near the Γ point we prove that a lattice dynamical instability originates in an unstable acoustic phonon mode. The calculation of the elastic constants shows that the phase transition is a proper ferroelastic phase transition occurring under a homogenous deformation of the crystal lattice. These results further illustrate the behavior of fluoride materials under extreme conditions, such as pressure.